A diffusion transfer film unit consists essentially of an image-generating layer (or a light-sensitive layer), a dye-receiving layer (or an image-receiving layer), and a processing solution, as described, e.g., U.S. Pat. No. 3,928,312. The image-generating layer contains a light-sensitive silver halide emulsion and releases a diffusing dye in proportion to an exposure. The dye-receiving layer has a function of fixing the dye generated in the image-generating layer. The processing solution contains at least a substance necessary for development, such as an alkali or a reducing agent.
Known diffusion transfer light-sensitive materials are roughly divided into a peel-apart type and a monosheet type (non-peel type).
The peel-apart type film unit has a light-sensitive layer and a dye-receiving layer on separate supports. After imagewise exposure, the light-sensitive element and the image-receiving element are brought into contact, a processing solution is spread therebetween, and the dye image-receiving element is peeled off to obtain a dye image transferred on the dye image-receiving layer. The peel-apart type film unit is disadvantageous in that the dye contained in the light-sensitive element would further be transferred to the dye image-receiving layer to disturb the color balance or to stain the white background unless the dye image-receiving element is peeled within a prescribed time.
The monosheet type film units comprise a pair of substrates one of which is transparent, in between which a dye image-receiving layer and a silver halide emulsion layer are provided. The dye-image receiving layer and the silver halide emulsion layer may be provided on the same transparent support or separate supports. In the former case, a white reflecting layer is provided between the image-receiving layer and the silver halide emulsion layer, and in the latter case a processing solution to be spread between the image-receiving layer and the silver halide emulsion layer contains a white pigment, so that the dye image transferred to the image-receiving layer may be seen by reflected light.
The monosheet type film units are disadvantageous in that it takes time for the dye released from the silver halide emulsion layer to be transferred to the image-receiving layer and be fixed thereon completely. It follows that the image undergoes great change in density with time after processing.
The above-mentioned problems either of the peel-apart type or the monosheet type tend to become conspicuous with an increasing amount of a dye generated. From this viewpoint, it has been demanded to establish a dye transfer technique by which a high transfer density can be obtained, processing time dependency is reduced, and change in density with time after processing is suppressed.
Retarded dye transfer causing various disadvantages as stated above should be minimized for improvement of image quality. To this effect, it has been proposed to provide a dye-trapping layer in a film unit as disclosed in British Patent 1537079, JP-B-60-2654, JP-A-50-142233, and JP-A-3-53248 (the terms "JP-A" and "JP-B" as used herein mean an unexamined published Japanese patent application and an examined published Japanese patent application, respectively). Although a dye-trapping layer is effective to capture the residual dye to suppress retarded dye transfer, it deteriorates green preservability of the film unit.
On the other hand, U.S. Pat. No. 3,928,312 discloses a color diffusion transfer process using an azo dye-forming substance which, upon being developed under a basic condition, releases an azo dye having different diffusibility from the image-forming dye. However, the azo dye-forming substance proposed does not always attain a high dye release efficiency, failing to obtain a sufficient transfer density.
Countermeasures against the above problem include the method described in JP-B-4-13701, but the method achieves no improving effects. In addition, the substances used in this method are accompanied by reduction in image stability.
As described above, a diffusion transfer process involves several problems attributed to the use of diffusing dyes, though superior in simpleness and rapidness in obtaining a photographic image.
The most serious of these problems is retarded dye transfer. In order to finish image formation within a reasonably short time, an image-generating layer must generate a dye in excess over the amount of the dye to be fixed on a dye-receiving layer. As a result, even at the time when transfer of a desired image is completed, an excess dye remains in a layer other than the dye-receiving layer. Retarded dye transfer is a phenomenon that the residual dye is gradually transferred to the dye-receiving layer and fixed thereon.
Retarded transfer in a monochromatic dye (for example, a black dye) transfer diffusion process makes the image harder than necessary, tending to lose the shadow gradation (i.e., plugging). Retarded transfer in a color diffusion transfer process based on a subtractive color process causes not only black plugging but disturbance of color balance because three kinds of dyes (for example, a yellow dye, a magenta dye and a cyan dye) are usually different in diffusibility and therefore different in requisite excess to be generated.
Further, the residual dye transferred to the image-receiving layer also diffuses to the planar direction to reduce the image sharpness.